1. Field of the Invention
The present invention relates to a novel process for producing a conjugated diene containing a cyano group having the formula ##EQU2## wherein R.sub.1 and R.sub.2 represent a hydrogen atom, an alkyl group or a phenyl group.
2. Description of the Prior Art
These compounds produced by the process of this invention are useful as monomers or comonomers for preparing plastics or elastomers. The conjugated diene having a cyano group wherein R.sub.1 and R.sub.2 are hydrogen atoms, i.e., 2-cyano-1,3-butadiene, is especially useful. Various schemes for syntheses have been proposed. Typical of these are:
1. Preparing methyl vinyl ketone cyanohydrin by reacting methyl vinyl ketone with hydrogen cyanide, and thereafter acetylating to produce 3-acetoxy-3-cyano-butene-1. 2-Cyano-1,3-butadiene is then produced by the thermal decomposition of the product at 550.degree.-570.degree.C [disclosed in B.P. 482,300, J. Am. Chem. Soc. 70, 1775 (1948) and U.S. Pat. No. 2,205,239].
2. Methyl vinyl ketone cyanohydrin is selectively dehydrated at 400.degree.-700.degree.C in the presence of phosphoric acid. [disclosed in West German Pat. No. 1,081,006 and West German Pat. No. 1,113,216.]
3. Preparing 2-cyano-1,3-butadiene by dehydrocyanation of 1,2-dicyano-cyclobutane in the presence of a solid alkaline oxide catalyst. [disclosed in U.S. Pat. No. 3,347,902.]
However, the present invention for producing 2-cyano-1,3-butadiene and similar compounds is quite different from the abovementioned conventional processes.
The 2-cyano-3-hydroxy-1-olefin compounds used as the starting material of this invention are alcohols. In a synthesis of an olefin by the dehydration of an alcohol, it is customary to use as a catalyst a Bronsted acid, e.g., sulfuric acid, phosphoric acid, or a Lewis acid, e.g., zinc chloride, iodine, together with either an acid anhydride, e.g., boric anhydride, phthalic anhydride, acetic anhydride or phosphorus pentoxide, phosphorous trichloride, etc. as a dehydrating agent. It is also well-known to use alumina, silica gel or thorium oxide as carriers for solid catalysts. Consequently, it might be expected that a conjugated diene containing a cyano group could be produced by the dehydration of the starting compound in the presence of such an acidic catalyst. To the contrary, it has been found unexpectedly that such a dehydration is inhibited in the presence of an acid. Experimental acid dehydrations of 2-cyano-3-hydroxy-1-olefin compounds either could not be effected or produced only quite small yields. When a solid acidic catalyst is used, it has been found that a small amount of the cyano-containing conjugated diene is produced at the initiation of the reaction. However, this could not be put to industrial use because only thermal decomposition of the compound is involved.
It has been attempted to produce conjugated dienes containing a cyano group from 2-cyano-3-hydroxy-1-olefins using other schemes. As a result, it was found that 1,8-diaza-bicyclo[5,4,0] undecene-7 has catalytic action for the dehydration of 2-cyano-3-hydroxy-1-olefin compounds. As a result of further studies, the unexpected fact that catalytic dehydration of 2-cyano-3-hydroxy-1-olefin compound can be effected by using an alkaline material was discovered. To our knowledge, no other similar reaction, i.e., dehydration of secondary alcohol by alkaline catalyst, has been reported. Evidently, catalytic dehydration of 2-cyano-3-hydroxy-1-olefin compounds is a very unique dehydration, because the closely related compounds having the formula ##EQU3## wherein X represents ##EQU4## and R.sub.3 -R.sub.6 represent alkyl groups, could not be converted to the corresponding conjugated dienes by using an alkaline catalyst. It would be most desirable to take advantage of this novel finding to provide an alternate scheme for producing the cyano-containing conjugated dienes.